The present invention relates to the advantageous use of high sulfur content coal to generate power.
The burning of coal to generate power is regulated by the Environmental Protection Agency. Under current regulations coal having a sulfur content of greater than only 1% may not be burned unless the power plant is equipped with a large scrubber to remove SO2 from the exhaust and then the sulfur in the coal must not be more than 1.8%. Coal having no more than 1% sulfur is termed Compliance coal. Naturally occurring coals may have a much higher sulfur content. For instance, Appalachian and Ohio River Basin coal may have a sulfur content as high as 3-5%, occurring in seams of predominantly in the form of iron sulfide particles and its natural directive forms of iron and sulfur. These sulfur compounds are in the form of small nuggets and found in thin layers throughout the coal seams. When strip mining coal with high-sulfur seams, some high sulfur content coal may be mined along with low sulfur content coal. In addition, high-sulfur seams may have to be displaced to get at the low-sulfur coal lying underneath. The mined high sulfur content coal, if saleable, does not command a high price, at a substantial cost to the mine; and if unsaleable must be disposed of according to strict standards.
Many methods have been developed to decrease the sulfur content of coal. Most of these methods involve grinding or crushing the coal to release the small particles or nuggets of sulfur compounds, and, using the difference in density between the coal (density: 1.20-1.35 specific gravity) and the high sulfur compounds, such as iron sulfite (density=5) to separate the two. In the typical method, the small particle size coal and nuggets of sulfur compounds (with water from the crushing and grinding step), are spun in a centrifugal separator or hydrocyclone to xe2x80x9cspin downxe2x80x9d the reclaimed coal and water while xe2x80x9cspinning outxe2x80x9d the high sulfur compounds, and other rocks. Given the cost of Compliance coal, these methods are under constant development.
U.S. Pat. No. 3,926,787 describes an improved hydrocyclone with a three-tiered vortex-producing cone to more efficiently xe2x80x9cdrainxe2x80x9d the reclaimed coal at the vortex. The patent makes no mention of electronic controls, or continuous operation with varying sulfur content coal.
U.S. Pat. No. 5,571,490 discloses a chemical treatment for lowering the sulfur content of coal. According to this method, calcium carbonate is mixed with the high sulfur content coal, and reacts with the FeS2 to form CaSO4. In theory, chemical treatments are good, but are difficult to run, as they tend to overload the system, especially the scrubbers. In general, crushing or grinding, and spin separating methods are preferred.
In the past, these crushing and separating processes have been undertaken at the mining site, to produce low-sulfur, saleable, coal. This reclaimed coal was stored, and then transported to a power generating plant. This produced its own set of costs in the form of off-setting risks. To reclaim as much coal as possible, grinding is maximized, decreasing particle size, for better separation, and producing coal xe2x80x9cdustxe2x80x9d or fines. Dry coal fines are potentially explosive, presenting hazards in storing or transporting the fines. In addition, the water needed in the typical grinding and separating processes increases as the particle size of the ground coal decreases, producing moist coal fines, which are subject to freezing, yielding a non-flowable mass not easily loaded to, or unloaded from transport containers. De-sulfuring the coal at the mine also requires a batch process.
The continuously varying sulfur and moisture content coals arriving at the coal-fired power generating plants require careful monitoring of moisture levels to maintain an efficient burn and to properly desmoke the whole operation. U. S. Pat. No. 3,600,676 describes a method and apparatus for continuously determining the moisture content of bulk goods, such as coal. According to the method two capacitor electrodes or plates are placed adjacent to a flow of bulk goods, and the moisture content calculated as the ratio of contained moisture (from static losses between capacitor plates) to the quantity of the bulk materials (from the dynamic losses between the capacitor plates).
U.S. Pat. No. 5,272,745 describes a method and apparatus for determining the moisture content in a moist material, by measuring the cooling effect at a temperature above the boiling point of the liquid, and at a temperature below the boiling point of the liquid. This method is most appropriate for mixing road surfacing materials, such as cement.
U.S. Pat. No. 4,817,021 discloses a method and apparatus for measuring the water content, density and thickness of a layer of feedstock by measuring the impedance, radio frequency loss or dielectric loss, and back scattered gamma rays. This method, and its implementing apparatus are useful in the present invention. In addition, the patent describes a number of methods (at col., line 51 to col.2, line 5) for the measurement of moisture in coal using capacitance techniques.
U.S. Pat. No. 4,964,734 describes a method for the fast measurement of the moisture content for on-line processing. The method relies on not decomposing the sample with (microwave) heat, hence it requires the pre-measurement of a drying time for the sample material whose moisture is to be measured. Thus it does not lend itself to continuous processes with varying input. The patent also discloses prior art moisture content measuring processes, such a JIS M8811 (Japanese Industrial Standard), infrared moisture meters, electrostatic capacity type moisture meter, a microwave moisture meter, and a neutron moisture meter.
According to the present invention, a more efficient de-sulfuring of coal would take place at the power plant. This would allow combining a continuous de-sulfuring process with a continuous power generation process. With co-ordinated electronic blending controls the overall process can be streamlined to produce the most economical feedstock, and to maximize the economics for using varying sulfur and moisture content coals.
The present invention comprises both a method and apparatus, which provide a system for the reduction of the sulfur content of coal for either blending with compliance coal at the generating plant or for burning directly without being blended. With the method and apparatus of the present invention, the high sulfur content coal is ground, or crushed, and the sulfur compounds separated from the reclaimed coal, on site at the power generating plant. The result of the separation process is a continuous supply of moist ground coal, with a moisture content permissible for immediate use as feedstock at a power plant. In addition, the method and apparatus use electronic controls to maximize the economics of reducing the sulfur content, and of generating power. Hence, the present invention is directed to Compliance coal production at the power plant, in a sulfur reduction process that is electronically controlled and continuous, and whose end product is feedstock. By moving de-sulfuring of the coal to the power plant, and providing the coal and coal fines in a continuous supply of feedstock, the costs and the risks of transporting and using the fines is reduced. In addition, the carefully electronically controlled production of feedstock permits calculation of economies of coal use at the time of power generation therefrom.